Persistent feelings of fatigue are a widespread complaint reported by older adults, and are associated with detriments in health and quality of life. Aging is also accompanied by gains in adiposity, reductions in physical activity, and loss of lean mass and physical function. This study assessed the influences of body composition [adiposity (%Fat) and mineral-free lean mass (MFLM)] and physical activity (total and moderate-to-vigorous; MVPA) on fatigue and function in older adults. Furthermore, we sought to determine the mediating role of systemic inflammation on these health outcomes. One-hundred eighty-two community-dwelling older adults were recruited to participate in this study (age = 69.2±6.7 years, 98 men, 84 women) Body composition [adiposity (%Fat) and relative muscle mass (Skeletal Muscle Index (SMI); appendicular MFLM/ht2] was quantified via dual-energy X-ray absorptiometry (DXA). Physical activity (PA) was assessed by the Physical Activity Scale for the Elderly (PASE), and accelerometers were worn to determine total and MVPA. Fasting blood samples were obtained for measurement of serum C-reactive protein (CRP), Interleukin-6 (IL-6), the soluble IL-6 receptor (sIL-6R) and WBC count. Fatigue was assessed by the Multidimensional Fatigue Inventory (MFI), to determine levels of general, physical and mental fatigue as well as reduced activity and motivation. Lower-extremity physical function (LEPF) was evaluated by 7-m walk tests, a Timed Up and Go (Up&Go), a 30-second chair stand test (30-Chair), a 6-min walk, the Short Physical Performance Battery (SPPB) and the Star-Excursion Balance Test (STAR). It was hypothesized that 1) women would report higher levels of fatigue and have poorer performance of physical function than men, 2) adiposity would be an independent predictor of fatigue and LEPF, 3) PA would be inversely related to fatigue and positively with LEPF, and 4) inflammation would be positively associated with fatigue. Understanding the biological and behavioral influences on fatigue and function is imperative for combating health detriments in older adults.
Men and women reported similar levels of fatigue in all dimensions (p > 0.05) with the exception of women reporting higher levels of mental fatigue than men (p = 0.05). Adiposity was positively correlated with several measures of fatigue (r range = 0.20 to 0.42), whereas PA was inversely associated with the same measures of fatigue (r range = 0.18 to 0.37), both of which were not related to mental fatigue. CRP, IL-6 and WBC were also related to several dimensions of fatigue (r range = 0.15 to 0.26). Compared across PA-adiposity groups, in the absence of an interaction, there was a main effect of PA and adiposity on general and physical fatigue (p < 0.05). Regression analyses revealed that the psychosocial variables depression and sleep quality and adiposity independently explained variance in general and physical fatigue. Adiposity and inflammation are positively related to general and physical fatigue, with adiposity remaining a predictor of both dimensions, independent of other associated factors. In addition, PA is inversely associated with these same dimensions of fatigue, and is an independent predictor of mental fatigue. Adiposity, physical activity and inflammation are identified as potential targets for reducing fatigue in older adults.
Men performed better on all LEPF tests than women (all p < 0.05). Unlike all other independent variables, MFLM and SMI were not related to any LEPF outcomes. In the absence of a significant interaction, main effects for adiposity were found for mobility tests of LEPF, including WALK, Up&Go, 30-Chair and 6-min walk. There was a main effect of PA on 6-min walk only with greater PA corresponding to better performance. On STAR balance tests, an interaction existed for medial, posterior and composite reaches (p < 0.05). After accounting for influences of sex, age and number of co-morbidities, %Fat remained a significant predictor of all mobility measures and the STAR composite, as did PA for Up&Go and 6-min walk (all p < 0.05).
Our results suggest that adiposity is a major determinant of fatigue and both balance and gait-related physical function in relatively healthy older adults. Physical activity may help to prevent age-associated fatigue and loss of mobility; however there does not seem to be an added benefit of MVPA.
Given the high prevalence and associated health detriments of fatigue in older adults, longitudinal studies involving reductions in adiposity and increasing physical activity as possible prevention and treatment strategies for both fatigue and mobility impairments are warranted. Due to the complexity of these relationships, future work should simultaneously assess body composition components, physical activity and inflammation to further our understanding the disablement process. Uncovering the key influential factors contributing to fatigue and disability is essential for development and implementation of effective prevention and treatment strategies.